content: Simplify RenderWidgetCompositor construction

Currently it uses the LayerTreeHostClient to pass a bool around from
RenderWidget to RenderWidgetCompositor, and constructs the cc
structures in RenderWidget, when RenderWidgetCompositor is meant to
be wrapping them, so it could just create them.

But expose the cc::LayerTreeSettings to RenderWidget so that tests
have a chance to override those, instead. They don't need to be able
to replace the LayerTreeHost/AnimationHost types.

NOTRY=true
TBR=pdr

Bug: 838693
Cq-Include-Trybots: luci.chromium.try:android_optional_gpu_tests_rel;master.tryserver.blink:linux_trusty_blink_rel
Change-Id: If7742e248b3d8799d1590dee43ce2eeb7e516146
Reviewed-on: https://chromium-review.googlesource.com/1112595
Commit-Queue: danakj <danakj@chromium.org>
Reviewed-by: danakj <danakj@chromium.org>
Reviewed-by: Antoine Labour <piman@chromium.org>
Reviewed-by: enne <enne@chromium.org>
Cr-Commit-Position: refs/heads/master@{#570438}

• cc/test/layer_tree_test.cc[diff]
• cc/test/stub_layer_tree_host_client.cc[diff]
• cc/test/stub_layer_tree_host_client.h[diff]
• cc/trees/layer_tree_host_client.h[diff]
• content/browser/renderer_host/compositor_impl_android.cc[diff]
• content/browser/renderer_host/compositor_impl_android.h[diff]
• content/renderer/gpu/render_widget_compositor.cc[diff]
• content/renderer/gpu/render_widget_compositor.h[diff]
• content/renderer/gpu/render_widget_compositor_unittest.cc[diff]
• content/renderer/render_widget.cc[diff]
• third_party/blink/renderer/platform/testing/web_layer_tree_view_impl_for_testing.cc[diff]
• third_party/blink/renderer/platform/testing/web_layer_tree_view_impl_for_testing.h[diff]
• ui/compositor/compositor.cc[diff]
• ui/compositor/compositor.h[diff]